Field of the Invention
The present invention relates to a sensing method for measuring an intensity of a magnetic field, specifically to an optically pumped magnetometer using atomic electron spin or nuclear spin.
Description of the Related Art
An optically pumped magnetometer measures a rotation, by a measured magnetic field, of a spin of an atom group polarized by a pump light, as a rotation of a polarization plane of a probe light.
As such an optically pumped magnetometer, the specification of U.S. Pat. No. 7,038,450 describes an optically pumped magnetometer having a cell containing alkali-metal gases, a pump light source, and a probe light source to have an ability to detect weak magnetic fields. In this specification of U.S. Pat. No. 7,038,450, a probe light is made to enter in a direction perpendicular to the optical path of a pump light. Further, in “Spin-exchange relaxation-free magnetometry using elliptically polarized light,” Phys. Rev. A 80, 013416 (2009), V. Shah and M. V. Romalis describe an optically pumped magnetometer in which a single-axis elliptically polarized light is made to enter a cell to increase the degree of flexibility of the device layout.
Further, in the specification of U.S. Pat. No. 6,318,092, a technique for the purpose of polarizing 3He, though not for the purpose of use in a magnetometer, is reported, in which spin exchange interaction between Rb and K is used to transfer the rubidium (Rb) spin polarization to the potassium (K). In the technique, the Rb is optically pumped to polarize the K, and the 3He is finally polarized. Using this spin exchange interaction, also, the K can be polarized sufficiently like in that of optical pumping.
However, the above-mentioned conventional examples of optically pumped magnetometers have the following problems: In the specification of U.S. Pat. No. 7,038,450, since the pump light and the probe light are arranged orthogonally, a flexibility of the device layout is restricted. Particularly, since optical axes of the pump light and the probe light interfere with array-arranged cells, such an array-arrangement of multiple cells is difficult. Further, in the above technique described by Shah et al., the probe light is elliptically polarized to create a layout to have the same action as in a layout in which a circularly polarized pump light and a linearly polarized probe light are concentrically overlapped. However, in measuring the rotation angle of the polarization plane of the probe light, it is difficult to separate between a component acting as the pump light and a component acting as the probe light, and hence optical shot noise due to a component acting as the pump light cannot be separated from a signal, thereby causing a problem that a noise level increases.